1. Field of the Invention
A method for detecting the presence of corrosion of a structure (e.g., wall, container, vessel, tank, or pipe) using a magnetically coupled sensing system that remotely monitors the health of one or more corrosion coupons. It is best used when physical access to the coupon is difficult, costly, or impossible.
2. Description of the Prior Art
Corrosion will reduce the useful life of a structure. Corrosion may result in the thinning of the structure, pitting of the structure, or cracking of the structure. The type of corrosion that may occur and the type of corrosion monitoring systems needed to assess the degree of corrosion will depend on the service environment of the structure and the condition and operational use of the structure. There are three basic approaches to corrosion monitoring. The first is to make a “direct” measurement of the physical properties of the structure itself. The second is to use a “surrogate” material positioned in the service area, which is identical to the material in the structure, and infer the corrosion of the structure from the surrogate material. The third is to monitor the “chemistry” of the solution or gas upstream, downstream or within the service environment and then infer the effects of corrosion on the structure from an empirical or theoretical relationship that relates the measured quantity to the corrosion-induced damage.
The objective of all three corrosion-monitoring methods is to predict the remaining useful life of the structure of interest from an estimate of the corrosion measured or inferred with the monitoring method. In the case of monitoring the structure directly, a simple extrapolation can be made once several time-sequenced measurements have been made. In the case of either monitoring corrosive chemistry or monitoring surrogates, an inference must be made that correlates the corrosion measurement taken to the actual impacts on the structure.
Direct monitoring is a preferred method, but due to access, safety, or cost implications, this approach is not always viable. Direct monitoring may involve visual or photographic inspection of the structure, or physical measurements of the dimensions of the structure (e.g., (1) wall thickness; (2) pit depth, diameter or pit density; or (3) crack depth, width, length or density). For many structures in which one side of the structure is accessible, the use of non-destructive examination equipment such as ultrasonic or eddy current techniques can be used to provide general wall thickness data or cross-sectional imaging. The main problem with direct monitoring is the access to the structure is needed and in many instances, access is not possible. Such measurements cannot be practically be made, for example, in radioactive storage containers, or on the walls of underground or the floor of aboveground storage tanks and piping containing petroleum or other hazardous substances and hazardous waste. For these types of applications, surrogate monitoring and chemistry monitoring systems are normally employed.
There are commercially available corrosion monitoring techniques that involve direct monitoring of a surrogate. The surrogate material is typically made of the same material as the structure of interest. The most common surrogate monitoring approach is the direct placement of corrosion coupons in the environment of interest. Corrosion coupons are the lowest-tech method of corrosion monitoring via surrogates. A corrosion coupon is a piece of material similar (identical) to the material of interest. The corrosion coupon(s) are placed in similar service conditions and then removed from the service area and evaluated at a later date. These coupon inspections are done periodically and are not linked to a specific level of corrosion. The coupons may be analyzed using destructive metallography. They may be inspected for appearance and/or weighed and compared to the pre-service weight do determine material loss. The use of corrosion coupons, while viewed as a very good method of assessing corrosion, is typically expensive and inconvenient to use. In some instances, the structure needs to be taken out of service to remove the coupons from the service area, which is expensive and may have health and safety implications. As presently used, corrosion coupons do not give any early warning of impending failure until they are retrieved and examined.
Electrical Resistance (ER) and Linear Polarization Resistance (LPR) probes both rely on electrical current being passed through a surrogate material and measuring changes in the resistance of the electrical circuit as the surrogate material degrades. Essentially, current is passed through a known cross-section; as metal disappears, resistance increases. Both ER and LPR probes are effective means for measuring uniform corrosion; however, correlating the change in resistance of an ER or LPR probe to the physical changes to the structure caused by corrosion can be imprecise and not yield good answers for many applications.
Developed for, and applied at, the U.S. Department of Energy's (DOE's) Hanford tank farms, electrochemical noise corrosion probes measure corrosion current and potential (voltage) between three surrogate electrodes. The relationship between corrosion current and corrosion potential on each electrode is indicative of the type and magnitude of corrosion on the electrodes, which can then infer the type and magnitude of corrosion on the structure. While electrochemical noise is a viable technology for early warning of stress corrosion cracking and pitting, its ability to quantify corrosion in a new application requires confirmatory laboratory corrosion studies in order to reliably correlate corrosion probe data with degradation of the structure.
A Thin-Wall Membrane Corrosion Probe is a one-shot vacuum chamber with a thin-wall membrane and a sensor. When the thin wall is breached by a through-wall pit, a signal is generated. This device operates somewhat like a balloon; when the balloon is “popped”, the pressure change is used to indicate the breach. This device is an excellent pitting corrosion detector.
The present invention describes a method and apparatus for remotely and automatically determining the amount and rate of corrosion of a structure or the material in the structure in a difficult to access environment without the need to handle or remove the corrosion coupon from the environment. The patent literature does not describe any such invention using corrosion coupons. U.S. Pat. No. 4,120,313 describe holding and/or handling systems for corrosion coupons. There are however, numerous inventions in the patent literature that describe electrical noise, electrical resistance and linear polarization methods and apparatuses. For example, U.S. Pat. Nos. 3,609,549; 3,936,737; 4,181,882; 4,238,298; 5,139,627; 5,446,369 describe such inventions.
In U.S. Pat. No. 6,499,353, Douglas, et. al., describes a magnetically coupled pressure gauge that measures the pressure or temperature inside a seal container and generates magnetic signal outside the container that yields a continuous measurement of pressure or temperature. In U.S. Pat. No. 5,284,061, Seeley, et. al., describes an apparatus for measuring pressure change of a specified amount in a sealed container that is mainly intended to detect a gas leak due to a loss of pressure. In U.S. Pat. No. 6,182,514, Hodges describes a pressure monitoring system for seal containers using bellows and magnet to monitor pressure. In U.S. Pat. No. 6,067,855, Brown, et. al., describes an apparatus for measuring liquid level in a container, which communicates the level changes of a float riding on the liquid surface to the outside of the container using magnetic sensing strip. None of these systems monitor corrosion and none of these systems use corrosion coupons.
The present invention was initially conceived to address a potential corrosion problem in a sealed stainless steel container holding radioactive material in a specialized container system known as a 3013 canister. However, the invention has extensive application to corrosion monitoring in general. It can be used to monitor corrosion in storage tanks and pipelines containing liquids and gases that may be corrosive to the walls of the tank or pipe. It has the potential for use in many less obvious application like furnaces and other structures where access is difficult.
API 653 requires the floor of an aboveground storage tank containing petroleum products be periodically inspected. The time between inspections can be increased and the inspections improved if the rate of corrosion of the floor or inside walls of the tank can be measured. The same is true for pipelines.
The present invention automates the use of corrosion coupons and mitigates the common and important disadvantages this approach. The coupon does not need to be removed from the service area for assessment, and periodic assessments are not required. Also, the present invention does not disturb the service environment, which occurs when coupons are removed. More importantly, the present invention indicates when a certain specified level of corrosion occurs. A time sequence of measurements can be made using multiple coupons. Coupons with different physical characteristics and/or loadings can be used to determine different types and different levels of corrosion. For example, a thin coupon can be used to indicate that corrosion is occurring, but at a level of negligible impact to the structure. A thicker coupon can be used to give an early warning of an important level of corrosion and may indicate that a more thorough inspection of the structure is required. Finally, an even thicker coupon may indicate that the structure needs replacement or upgrading.
In view of the prior art described above, it is apparent that there is a need and a wide range of applications for a method and apparatus that can remotely and automatically measure corrosion using coupons without requiring the removal of the coupons from the service environment.